The proton-sucrose symporter mediates the key transport step in the resource distribution system that allows many plants to function as multicellular organisms. In the results reported here, we identify sucrose as a signaling molecule in a previously undescribed signal-transduction pathway that regulates the symporter. Sucrose symporter activity declined in plasma membrane vesicles isolated from leaves fed exogenous sucrose via the xylem transpiration stream. Symporter activity dropped to 35-50% of water controls when the leaves were fed 100 mM sucrose and to 20-25% of controls with 250 mM sucrose. In contrast, alanine symporter and glucose transporter activities did not change in response to sucrose treatments. Decreased sucrose symporter activity was detectable after 8 h and reached a maximum by 24 h. Kinetic analysis of transport activity showed a decrease in V max . RNA gel blot analysis revealed a decrease in symporter message levels, suggesting a drop in transcriptional activity or a decrease in mRNA stability. Control experiments showed that these responses were not the result of changing osmotic conditions. Equal molar concentrations of hexoses did not elicit the response, and mannoheptulose, a hexokinase inhibitor, did not block the sucrose effect. These data are consistent with a sucrose-specific response pathway that is not mediated by hexokinase as the sugar sensor. Sucrosedependent changes in the sucrose symporter were reversible, suggesting this sucrose-sensing pathway can modulate transport activity as a function of changing sucrose concentrations in the leaf. These results demonstrate the existence of a signaling pathway that can control assimilate partitioning at the level of phloem translocation.Resource allocation between tissues is a fundamental process in all multicellular organisms. In higher plants, leaves function as the principle site of resource acquisition by utilizing the free energy captured in photosynthesis for the reductive assimilation of oxidized forms of carbon and nitrogen into carbohydrates and amino acids, respectively. These resources are subsequently distributed to the many heterotrophic tissues of the plant. Indeed, as much as 80% of the carbon acquired in photosynthesis is transported in the plant's vascular system to the import-dependent organs. Sucrose is arguably the most important metabolite in this system of resource allocation because it is generally the major end product of photosynthetic carbon metabolism and, in most plants, it is the predominant form of carbon transported to the heterotrophic tissues (1, 2). Moreover, in many plants energy-dependent sucrose accumulation in the phloem generates the high hydrostatic pressure that drives the long-distance flow of resources. The systemic distribution of photosynthate is known as ''assimilate partitioning,'' and it is a major determinant of plant growth and productivity (3).Our understanding of assimilate partitioning has advanced considerably over the last 10 years with the successful biochemical and molecu...

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Interest in algae as a feedstock for biofuel production has risen in recent years, due to projections that algae can produce lipids (oil) at a rate significantly higher than agriculture-based feedstocks. Current research and development of enclosed photobioreactors for commercialscale algal oil production is directed towards pushing the upper limit of productivity beyond that of open ponds. So far, most of this development is in a prototype stage, so working

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The bacterial Sec and signal recognition particle (ffh-dependent) protein translocation mechanisms are conserved between prokaryotes and higher plant chloroplasts. A third translocation mechanism in chloroplasts [the proton concentration difference (⌬pH) pathway] was previously thought to be unique. The hcf106 mutation of maize disrupts the localization of proteins transported through this ⌬pH pathway in isolated chloroplasts. The Hcf106 gene encodes a receptor-like thylakoid membrane protein, which shows homology to open reading frames from all completely sequenced bacterial genomes, which suggests that the ⌬pH pathway has been conserved since the endosymbiotic origin of chloroplasts. Thus, the third protein translocation pathway, of which HCF106 is a component, is found in both bacteria and plants.

A proton-sucrose symporter mediates the key step in carbon export from leaves of most plants. Sucrose transport activity and steady-state mRNA levels of BvSUT1, a sugar beet leaf sucrose symporter, are negatively regulated specifically by sucrose. Results reported here show that BvSUT1 mRNA was localized to companion cells of the leaf's vascular system, which supports its role in the systemic distribution of photoassimilate. Immunoblot analysis showed that decreased transport activity was caused by a reduction in the abundance of symporter protein. RNA gel blot analysis of the leaf symporter revealed that message levels also declined, and nuclear run-on experiments demonstrated that this was the result of decreased transcription. Further analysis showed that symporter protein and message are both degraded rapidly. Taken together, these data show that phloem loading is regulated by means of sucrose-mediated changes in transcription of a phloemspecific sucrose symporter gene in a regulatory system that may play a pivotal role in balancing photosynthetic activity with resource utilization. P lants are photoautotrophic organisms composed of both heterotrophic (sink) and autotrophic (source) tissues. Resources acquired via photosynthesis in source leaves are loaded into the phloem of the plant's elaborate vascular system and distributed among the sink tissues in a process called ''assimilate partitioning''. Because up to 80% of all fixed carbon is exported to sinks (1), regulation of assimilate partitioning is vital to plant growth and development, and it can have dramatic affects on crop yield and productivity (2). Long-distance transport in the phloem cells of the plant vascular system is mediated by a positive hydrostatic pressure difference between the source and sink tissues that drives mass flow of solution. Positive pressure in the leaf phloem results from hyperaccumulation of an osmotically active solute. In most plants this solute is sucrose, which is loaded into the phloem by an electrogenic secondary active proton-sucrose symporter (3-5). Recently, the sucrose transport activity of a sugar beet leaf proton-sucrose symporter (BvSUT1) was found to be negatively regulated by sucrose and not by hexoses or changes in osmotic potential (6), but the mechanism and implications of this regulatory pathway remained unresolved. Here we report that sucrose transport activity in the phloem is regulated via sucrose-mediated changes in transcription of the proton-sucrose symporter gene in a regulatory system that plays a pivotal role in balancing photosynthetic activity with resource utilization. Materials and MethodsTissue Fixation and Embedding. Leaf tissue from mature sugar beets (Beta vulgaris Linnaeus) was fixed on ice for 6 h under vacuum in 4% wt/vol paraformaldehyde in potassium phosphate buffer (pH 7.4). Tissue was dehydrated through a 10% stepgraded ethanol series beginning at 20% and ending at 90% and allowed to incubate overnight at 4°C. After three 100% ethanol washes, the tissue was substituted with tertiary...

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We have identified a new amino acid transporter from the Arabidopsis thaliana expressed sequence tag cDNA collection by functional complementation of a yeast amino acid transport mutant. Transport analysis of the expressed protein in yeast shows that it is a high-affinity transporter for both lysine (Lys) and histidine with Michaelis constant values of 175 and 400 [mu]M, respectively. This transporter (LHT1, lysine histidine transporter) has little affinity for arginine when measured directly in uptake experiments or indirectly with substrate competition. The cDNA is 1.7 kb with an open reading frame that codes for a protein with 446 amino acids and a calculated molecular mass of 50.5 kD. Hydropathy analysis shows that LHT1 is an integral membrane protein with 9 to 10 putative membrane-spanning domains. Southern-blot analysis suggests that LHT1 is a single-copy gene in the Arabidopsis genome. RNA gel-blot analysis shows that this transporter is present in all tissues, with the strongest expression in young leaves, flowers, and siliques. Wholemount, in situ hybridization revealed that expression is further localized on the surface of roots in young seedlings and in pollen. Overall, LHT1 belongs to a new class of amino acid transporter that is specific for Lys and histidine, and, given its substrate specificity, it has significant promise as a tool for improving the Lys content of Lys-deficient grains.